1. Field of the Invention
The present invention relates to a three-phase brushless servo motor.
2. Description of the Related Art
FIG. 1 shows the configuration of a conventional three-phase brushless servo motor. In FIG. 1, the reference numeral 41 designates the brushless motor; 42, an encoder for detecting the rotational speed; 42a, a rotary disk which rotates in interlinkage with the brushless motor 41; 42b, slits formed in a rotary disk 42a; 42c, a light emission element; 42d, a light detection element; 142, an output signal of the rotational speed detecting encoder 42; and 43, a connector connecting the brushless motor 41 and the rotational speed detecting encoder 42 to each other.
The reference numeral 44 designates a motor driving circuit; and 144, an output of the motor driving circuit 44 which is a voltage to be applied to each of motor coils. The reference numerals 45a, 45b and 45c designate Hall elements which are arranged so as to be separated from each other by 120 degrees in electrical angle. The reference numeral 145 designates the output signals of the Hall elements. The reference numerals 46a, 46b and 46c designate motor coils which are arranged so as to be separated from each other by 120 degrees in electrical angle. The reference numeral 47 designates a magnet in which each of north and south (N and S) poles is provided in a fan shape; 48, a back yoke of the magnet 47; 49, a frequency-to-voltage converter for converting a frequency into a voltage; and 149, an output signal of the frequency-to-voltage converter 49.
FIG. 2 shows waveforms of signals at various portions of the above-mentioned three-phase brushless servo motor. In FIGS. 2, (a), (b) and (c) show the encoder output signal 142, the converter output signal 149, and the Hall element output signal 145 in FIG. 1 respectively, and (d-1), (d-2) and (d-3) each shows the motor coil application voltage 144 in FIG. 1 respectively.
Referring to FIGS. 1 and 2, the operation of the above-mentioned conventional three-phase brushless servo motor will be described. In FIG. 1, when the brushless motor 41 rotates, the rotary disk 42a inside the rotational speed detecting encoder 42 is rotated through the connector 43 so that the slits 42b successively pass between the light emission element 42c and the light detection element 42d. Accordingly, the rotational speed detecting encoder 42 generates a pulse waveform as shown in (a) of FIG. 2. The frequency of this pulse signal is proportional to the rotational speed of the motor. By means of the frequency-to-voltage converter 49, the frequency of this pulse signal is converted into a voltage which is information of the rotational speed of the brushless motor 41 and which has a waveform as shown in (b) of FIG. 2.
Further, when the brushless motor 41 rotates, the Hall elements 45a, 45b and 45c generate Hall element output signals 145 which show three-phase signal waveforms which are shifted in phase from each other by 120 degrees in electrical angle as shown in (c) of FIG. 2. The Hall element output signals 145 are supplied to the motor driving circuit 44 so as to be used for timing setting of the output signal of the motor driving circuit 44, as shown in (d-1) of FIG. 2, that is, the motor coil application voltage 144. The level of the motor coil application voltage 144 is controlled by the converter output signal 149 which is rotational speed information so that the level is made low as shown in (d-3) of FIG. 2 when the motor speed is high, while it is made high as shown in (d-2) of FIG. 2 when the motor speed is low. According, the brushless motor 41 can keep its speed constant.
Thus, in the above-mentioned conventional three-phase brushless servo motor, a constant speed servo is realized by a combination of the rotational speed detecting encoder 42 and the brushless motor 41.
In the above-mentioned conventional brushless servo motor, however, there has been a serious problem that an expensive rotational speed detecting encoder is required so that the cost of the motor is high and the shape of the motor becomes large by a space corresponding to the detector and the connector.